This invention relates generally to optical systems and apparatus that exploit the decoupling of transverse modes in microstructure optical fibers (MOFs).
Microstructure optical fibers (MOFs) have recently been shown to exhibit large values of anomalous dispersion (positive D) for wavelengths above xcx9c700 nm. See, U.S. Pat. No. 6,097,870 filed on May 17, 1999 and issued on Aug. 1, 2000 to J. K. Ranka and R. S. Windeler (hereinafter the Ranka-Windeler patent) and J. K. Ranka et al., Optics Lett., Vol. 25, No. 1, pp. 25-27 (January 2000), both of which are incorporated herein by reference. In the 600-1100 nm wavelength range MOFs allow for a myriad of nonlinear effects that have previously not been possible or have been severely limited due to the large normal dispersion (negative D) of standard silica fibers. These effects include broadband continuum generation, four-wave mixing, and pulse compression.
In nonlinear optical interactions such as sum-frequency generation and four-wave mixing, the efficiency of the process depends strongly on the wave-vector mismatch, also known as the phase mismatch, between the pump and signal waves. See, for example, Agrawal, Nonlinear Fiber Optics, Academic Press (1995), which is incorporated herein by reference. In optical fibers, phase matching can be achieved by using either birefringence, where the pump and generated signals are in different polarization states, or by using multimode fibers, where the pump and generated signals are in different transverse spatial modes. One limitation to multimode phase matching is that random fluctuations and perturbations of the fiber will mix the various generated modes, reducing the efficiency of the process and generating a spatially incoherent output.
Thus, a need remains in the art for an optical fiber in which the transverse spatial modes are essentially decoupled from one another even in the presence of significant perturbations.
Such a fiber would allow not only for efficient nonlinear optical interactions in processes of the type discussed above, but also for information to be impressed on individual spatial modes in an optical communication system.
We have found that a properly designed MOF can simultaneously exhibit large anomalous dispersion at visible and near infrared wavelengths and support numerous transverse spatial modes that are essentially decoupled from one another, even in the presence of significant perturbations. In a MOF that includes an inner cladding region comprising at least one thin layer of air holes surrounding a core region, the key is to achieve a relatively large wave vector mismatch between the lowest order modes by appropriate design of the size of the core region and of the effective refractive index difference between the core region and the inner cladding region.
The surprising result is that such a MOF can be made to appear to be single mode with propagation properties that heretofore could be achieved in multimode waveguides only.
In accordance with one aspect of our invention, MOFs are designed to exhibit simultaneously relatively large anomalous dispersion and essentially decoupled transverse spatial modes by making the diameter of the core region less than about 6 xcexcm and the difference in effective refractive index between the core and cladding regions greater than about 0.1 (10%). Preferably, the cladding region contains no more than 2 layers of air holes, and the distance between the nearest edges of adjacent air holes is less than about 1 xcexcm.
MOFs with these features enable several embodiments of our invention. One embodiment is a nonlinear optical system comprising an optical pump source, an optical signal source, a utilization device and an optical fiber transmission path that optically couples the sources to the device. The transmission path includes at least a section of MOF in which at least two transverse modes remain decoupled from one another over the length of the MOF section, the output of the pump source propagating in one of the transverse modes and the output of the signal source propagating in another of the transverse modes. Another embodiment is an optical transmission system comprising an optical transmitter, a utilization device and an optical fiber transmission path that optically couples the transmitter and the utilization device. The transmission path includes at least a section of MOF in which at least two transverse modes remain decoupled from one another over the length of the MOF section, and further includes a first modulator for impressing information on a first optical signal to be propagated along the MOF in one of the transverse modes and a second modulator for impressing information on a second optical signal to be propagated along the MOF in another of the transverse modes.